Consider making VisualStudioMetadataReferenceManager.ctor thread safe

The main thread affinity of this ctor should be investigated further to determine if it would be possible to remove that dependency. Tying this to the main thread puts a bit of an onus on the creators of this type to understand this requirement, and the main thread in VS is already bogged down quite a bit, particularly during package/solution load.

Start by locating VisualStudioMetadataReferenceManager.ctor and tracing the callers that require the main thread. Investigate its use during package and solution load, then determine whether the thread-affinity dependency can be removed safely; completion should include a documented conclusion and any necessary follow-up scope.

Written by the indexing model from the issue text.